The invention relates to a device to carry out measurements in a vacuum chamber, in particular to measure thin layers, with a case, exhibiting at least one measurement window, to receive a measurement system.
Furthermore, the invention relates to a vacuum adapter for mechanisms to carry out optical measurements in a vacuum chamber.
Measurements in the vacuum, in particular for quality control, are quite important, for example, in the production of semiconductors. The most frequent variables to be measured are, for example, the thickness of the thin functional layers ranging from a few nanometers to some micrometers and the size and distribution of the particles. With the transition to smaller structures and larger wafer diameters, the demand for process-accompanying quality control increases. The goal is the early detection of errors and the correction of process parameters to increase the yield and the productivity of the production process. The higher the requirements of the quality control are, the more often the wafers have to leave the production process in order to be subjected to a random measurement.
The state of the art includes the so-called stand alone measuring machines, which are installed at central points of the plant. Due to the high cost of these systems and the relatively high space requirement, only a few of these systems can be installed. One drawback is also the additional paths, the additional loading and unloading steps of the wafers from the transport boxes and back again. Moreover, much time is lost between the detection of a defect and the reaction, a state that can result in enormous losses as the process speed increases and the value of the individual wafers increases dramatically.
To improve the yield and to reduce the production cycles precisely in thin film production, the layer properties of the thin films should be measured as process oriented as possible. One preferred method for measuring the refractive index and the thickness of thin layers is ellipsometry. It exploits the change in the polarization state of the light after its reflection on the sample surface. In addition, the collimated and fully polarized light is focused on the sample at a specific angle of incidence. In addition to the reflection, the polarization state of the radiation changes as a function of the properties of the sample.
In the case of ellipsometric measurements for determining the coating parameters in production processes not only stand alone machines have been used, but there has also been an attempt to measure the coating parameters in situ. The EP 0 527 150 B1 proposes an arrangement for ellipsometric in situ measurements in an industrial coating system. The ellipsometer, according to the EP 0 527 150 B1, exhibits a so-called paddle, on which not only the wafers to be measured are disposed but also the analyzing and the polarizing unit as well as the beam deflecting units are disposed in the form of prisms. Both for the incident beam and for the reflected beam a tube is provided as the beam tube. In a preferred embodiment these two tubes are also fastened to the paddle.
This ellipsometer arrangement has grave drawbacks. To carry out the measurement with an adequately good angle of incidence (usually between 65 and 75 degrees), it is a drawback with respect to production engineering that the coating furnace, which was optimized for a high throughput, cannot be fully filled, because minimum spacing must be maintained between the wafer to be measured and its neighboring wafer. Since the coating furnace is filled differently than what would correspond to the optimized state, the flow conditions in the furnace change and thus the quality of the coating.
From the view point of measurement technology it is quite disadvantageous that the pumps, which are typical in a vacuum chamber, excite the tube and the paddle to self sustained oscillation, the results of which are falsified measurements. The high temperatures and gases, generated during production, have a negative impact on the measurement results, since prism wall deposits and thermally induced stresses can be expected.
The object of the invention is to provide a device to carry out the measurements in the vacuum. Said device ought not to exhibit either the drawbacks of the stand alone devices or the in situ measuring devices, but rather permit a process oriented measurement under optimal measurement conditions.
This problem is solved by a device, according to claim 1. In addition, the problem is solved by a vacuum adapter, according to claim 14.
The case of the inventive device is divided into two parts, of which the first part projects into the vacuum chamber or defines with at least one surface the vacuum of the vacuum chamber; and the second part of the case is located outside the vacuum chamber.
The vacuum chambers are not the process vacuum chambers, but, for example, transfer or lock chambers of the production system. Special measurement chambers, to which the inventive device can also be attached, can also be integrated into the production system. The samples to be measured are brought to the device for measurement purposes by means of the transport means, such as robots, which are present in the production system in any event. Then said samples are positioned in the immediate vicinity of the first part of the case.
The inventive device exhibits at least one adjusting device, which engages with the case and whose purpose is to change the position relative to the vacuum chamber or relative to the sample to be measured. Thus, it is guaranteed that the device can be adjusted prior to each measurement, a condition that can be necessary for many a measuring method. It is necessary especially when the demands on the measuring accuracy are high or when the positioning of the sample by the transport means in the process system is too inaccurate.
It is possible to adjust the device prior to every measurement without interrupting the production process by means of the means to dispose sealingly and moveably the case in the wall of the vacuum chamber. In addition, with these means the device can be uncoupled mechanically from the vacuum chamber. Furthermore, the device can be assembled and tested independently of the vacuum chamber.
The inventive device exhibits a counterpull device, which engages with the second part of the case and permits a force to act that acts against the force, acting on the device owing to the negative pressure of the vacuum of the vacuum chamber. Thus, the goal of a force-free and tension-free state of the device is achieved, a state that leads to a better operating mode of the individual components and a higher reliability and measuring accuracy. In particular, it makes it easier to adjust the device. Even the mechanical uncoupling of the device from the vacuum chamber is thus increased. A special advantage lies in the fact that the arrangement eliminates the need to use especially heavy duty motors for the adjusting device. Consequently small motors and correspondingly weak spindle drives, which have to be designed only to compensate for any reset forces of the means to dispose the case in a sealed and moveable manner in the wall of the vacuum chamber, are sufficient.
It has proved advantageous to install, among other things, bellows as the means for the sealing and moveable arrangement of the case in the wall of the vacuum chamber. Since the bellows do not exhibit any rigid expansion, they permit the position of the device to be adjusted in relation to the vacuum chamber or to the sample to be measured without having to accept any losses in the sealing effect. In addition, said bellows damp possible oscillations of the vacuum chamber relative to the device. In the event of lower vacuum and minimum adjustment paths, elastic seals are also conceivable.
To apply the counterforce, a spring suspension or a magnetic suspension is, for example, also conceivable. Preferred is a negative pressure chamber that is formed on the side of the device opposite the vacuum chamber and which is evacuated either separately or is connected to the large vacuum chamber of the production system by way of a vacuum connection so that the pressure is automatically compensated. In this second case the negative pressure chamber has a variable volume. Advantageously the measurement system is housed in the second part of the case.
In the measurement system there is an input signal, which is converted into a measurement signal through interaction with the sample. The input signal can be arbitrary electromagnetic waves. The input signal can be generated either directly in the measurement system or fed from outside the measurement system into the measurement system and can be, for example, monochromated or modulated there or modified differently to match the measurement conditions.
In addition, the measurement system includes at least one detector, which serves to detect the measurement signal. The evaluation of the measurement signal can take place in the measurement system. However, for the purpose of evaluation, the detection signal can also be sampled from the measurement system or stored inside or outside the measurement system.
The first part of the case serves to form the boundary between the vacuum in the production system and the atmosphere, i.e. usually air, in the measurement system in the second part of the case. The interface can be designed, for example, as a window, which is appropriate for a vacuum or a high vacuum and which is transparent for the input signal and the measurement signal.
Preferably the measurement system of the device, according to the invention, is designed in such a manner that it exhibits not only the at least one detector but also at least one light source or light feed, such as a fiber optic cable. Thus, optical measurements in the vacuum can be carried out with the device, according to the invention. The embodiment of the inventive device is appropriate especially for measurements on the surface of a sample, such as ellipsometric measurements to determine the thickness and the refractive index of coatings or also to determine the existing particles by measuring the scattered light or measuring the loss of the direct reflection on the sample or in a volume. These applications are appropriate especiallyxe2x80x94but not onlyxe2x80x94for the quality control in production processes in the semiconductor industry.
To carry out optical measurements in the vacuum, the first part of the case is designed as a vacuum adapter and exhibits a beam tube, through which the incoming beam is guided as the input signal, which was generated by the light source in the measurement system, and through which the emerging beam of the measurement signal is guided. The incoming beam does not have to be automatically generated by a light source inside the measurement system, but rather can also be generated outside the measurement system and be fed to the measurement system, for example, by way of light guide cables and emerge there as a light beam. The beam tube of the vacuum adapter is fastened with one end to that part of the case that contains the measurement system. By means of the vacuum adapter, it is possible to couple in, for example, an ellipsometer, a reflectometer and/or an FTIR [=Fourier transform infrared] spectrometer.
To ensure the desired measurement configuration or measurement geometry at the measurement site, the vacuum adapter exhibits advantageously a prism and/or a lens system on the end of the beam tube that faces the vacuum chamber. Beam properties, like divergence, dispersion and angle in relation to the sample, can be adapted to the specific features directly before the measurement and directly after the measurement. In lower vacuum it is possible to seal the prism or the lens system so as to be vacuum tight with the beam tube so that these optical elements act as interfaces between the atmosphere in the beam tube or the measurement system and the vacuum outside the device.
If the goal is supposed to be a very high measurement accuracy, the preferred solution is that such optical elements as the prisms or the lenses be located completely in the vacuum, since otherwise stress could be generated inside this optical system. The end of the beam tube that faces the measurement system exhibits advantageously a window, which is appropriate for a vacuum and is transparent to light of any wavelength. This vacuum window separates the atmosphere in the measurement system from the vacuum of the beam tube or the vacuum chamber. A vacuum prevails inside the beam tube.
Especially for ellipsometric applications of the device, it is advantageous to dispose the polarizer not necessarily in the measurement system, but rather to attach the polarizer on the beam tube interior or beam tube exterior of the prism system of the vacuum adapter. If a very high degree of polarization is relevant, the polarizer is attached preferably on the beam tube exterior of the prism system. Of course, for applications in high vacuum it can happen that, if it involves a polarization foil or the polarizer is attached by cementing on the prism system, the vacuum is affected negatively. In this case the polarizer is attached preferably to the beam tube interior.
Depending on the geometric features of the vacuum chamber, it can be necessary to design the beam tube of the vacuum adapter with a minimum cross section or curved. For these cases the beam tube of the vacuum adapter has advantageously deflecting prisms to guide the beam.
For the quality of the measurement results an accurate finding of the position of the sample in relation to the measurement beam is advantageous. To this end, the measurement system, disposed in the device of the invention, exhibits not only a measuring unit, which can be, for example, an ellipsometer, a reflectometer or a FTIR spectrometer but also an adjusting unit. The adjusting unit comprises at least one light source and at least one position sensitive detector. Preferably an adjusting laser is installed as the light source. Due to the installation of, for example, beam splitters, a light source, which may or may not also be present in the measuring unit, can be used simultaneously for the adjusting unit. The beam plane of the adjusting unit is moved parallel to the measurement geometry so that the system""s state of adjustment can be reproduced as correctly as possible.
In an especially preferred embodiment a triangulation configuration is used as the adjusting unit. Thus, both the sample spacing and the tilt can be measured automatically. In this case the adjusting unit comprises an adjusting laser, a beam splitter and two position sensitive detectors. The laser beam from the adjusting laser is split by means of the beam splitter into two beams that are moved so as to be parallel. The one beam is used to determine the sample spacing. When the spacing between the samples changes, the point of impingement on the sample surface and thus also the reflected beam""s point of impingement on one of the position sensitive detectors moves. The deviation from the desired value can be determined electronically and, if desired, can be used for an automatic correction by means of the servomotors. To determine the sample tilt, the second partial beam is used. When the sample tilts, this beam migrates to the second position sensitive detector, a state that can be determined in turn electronically. The tilt of the sample can be corrected mechanically, or the angle of tilt can be determined quantitatively and then taken into consideration during the evaluation of the measurement.
For tests, in which not only one point but also several points on the sample have to be measured, it has proved advantageous to use rotating sample tables. Between two measurements the sample can be further rotated about a specific angle by means of the rotating table. Preferably the rotating table is disposed on a linear table.
The linear table""s direction of movement is in the radial direction of the rotating table. Through a combination of translational and rotational motions, an even greater number of measurement points can be approached on the sample surface. Especially compact is a variant, where not the rotating table is disposed on a linear table, but deflecting prisms or mirrors are spaced at a specific distance from the rotating table so that they can be moved linearly in the radial direction of the rotating table. Through a combination of linear movements of the about the prisms or mirrors and the rotating movements of the rotating table, just as many measurement points can be approached as in the embodiment described above. [sic]
The device, according to the invention, exhibits many advantages. Since the inventive device permits the integration of the measurement process into the production process in that it is carried out between the process steps and there is no need for the sake of adjustment to intervene in the operating mode of the production system, the goal is reached, on the one hand, that the samples can be measured extremely process oriented and, on the other hand, that the measurements can be carried out in such a manner that production and measurement do not exert a mutual negative effect. At this stage it is now possible to measure arbitrary samples without disturbing the production process and to achieve thereby maximum measurement accuracy. Eliminated are the cost intensive intermediate paths between the individual process steps and the measurement and, above all, the long reaction time between the detection of production errors and the reaction thereto.
Since the measurement configuration or the measurement geometry can be uncoupled from the design of the measurement system by means of the vacuum adapter, the measurement system can be optimized for higher measurement accuracy in an economical design. In addition, the entire device can be easily adapted to the space conditions in already existing production systems. To retrofit already existing production plants, it suffices to introduce the vacuum adapter into the vacuum through an opening in the respective vacuum chamber.
Since the measurement system is not located under vacuum, maintenance work and retrofitting work can be done on the measurement system without having to ventilate the process line or the respective vacuum chamber. Even the expense of building the measurement system decreases, since it is not necessary to use vacuum appropriate components, such as cable ducts. In addition, the negative effect on the volume in the sample or measurement surroundings, for example, due to the electrical components is prevented.
Several devices of the invention can be integrated in such a manner at diverse points of the production process that they are adapted to the production process of the respective product. Since the retrofitting of the production systems with the inventive device is associated with little cost and the inventive device itself is economical due to its simple construction, there is the possibility of setting up a comprehensive quality control system inside the production process at a low investment cost.